Various typespec and compilation improvements

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/service/guards.ex

@@ -3,7 +3,31 @@ defmodule Mobilizon.Service.Guards do
   Various guards
   """
 
-  defguard is_valid_string?(value) when is_binary(value) and value != ""
+  @doc """
+  Returns `true` if `term` is a valid string and not empty.
 
-  defguard is_valid_list?(value) when is_list(value) and length(value) > 0
+  ## Examples
+
+      iex> is_valid_string("one")
+      true
+      iex> is_valid_string("")
+      false
+      iex> is_valid_string(2)
+      false
+  """
+  defguard is_valid_string(term) when is_binary(term) and term != ""
+
+  @doc """
+  Returns `true` if `term` is a valid list and not empty.
+
+  ## Examples
+
+      iex> is_valid_list(["one"])
+      true
+      iex> is_valid_list([])
+      false
+      iex> is_valid_list("foo")
+      false
+  """
+  defguard is_valid_list(term) when is_list(term) and length(term) > 0
 end